How to find the correct bulb and aftermarket brake light housing?

Are you trying to find the bulbs or are you asking about an aftermarket unit? If you mean you installed aftermarket lights or someone else did, if they were direct fit parts, they should use the same bulbs as the original lights did.
For your car the OE numbers for the rear brake lights and turn signals are 3157 bulbs. The reverse lights are either 3156 if it's the sedan or a 911 if it's the wagon or hatchback.
For the high mount brake light, it is a Wagner 17177, Phillips 12961 or equivalent.
Changing them depends on the vehicle, on the sedan the housing has to come out, on the wagon you remove the interior trim. The attached shows the process.
Did that cover the question, or did you mean something else?

Your car has some rather unique lights. You would need to look at the various sites and see if you can find any that would fit, a quick search only shows the OE style and one custom one. The Focus just isn't one of the "hot" cars for customizing, I guess. As for using LED replacements. You should be able to, but you would want the better versions not the low priced eBay specials because most of those do not put out enough light to work very well.

That's the hard part. I would opt for a name brand part like Philips, Wagner, Sylvania. But even then, they can be fussy and cause issues like rapid flashing or bulb failure indicators, then you install resistors to fix that and that just adds to the current draw, and you still use the same power as you were with a regular bulb, plus have the downsides of an LED in an incandescent fixture. It's why I generally don't tell people to use them. Instead, I look for complete units that use an LED board like the newer cars use. However, those are commonly only for the more popular cars.